Excessive GC pauses and ANR due to growing heap size when unread count gets >2600

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General information

• Version: 2.8.10+pcr
• Device: Motorola moto g(7) power
• Android Version: Android 9 (stock)
• Server name: theta.eu.org
• Server software: prosody trunk
• Installed server modules: CSI, MAM, others
• Conversations source: PlayStore Beta Channel

Steps to reproduce

1. Start Conversations fresh
2. Do things to raise the unread count above ~2600 (receive lots of PMs, be in a lot of active MUCs, have unread history, ...)

Expected result

• Conversations does not slow down or crash

Actual result

• Conversations' UI gets slower and slower, until it eventually throws an App Not Responding / quits

From the logcat output, it seems like having a lot of unread messages causes Conversations to use a lot of heap space. Eventually, it hits the Android limit and the OS starts forcing GC pauses on it in order to try and reclaim space, which doesn't work; the app becomes unusable, and eventually dies due to App Not Responding.

After restarting, the app only loads ~50 or so messages for each MUC / conversation, so it doesn't run into the issue.

Suggested action: maybe Conversations should evict unread messages from memory when the OS signals heap space is low? It looks like each Conversation keeps around a list of message objects and only gets rid of them after you've read them; it shouldn't be too bad to just deallocate said message objects when memory pressure is encountered, possibly with some logic for not deallocating objects that have caused a push notification.

Relevant debug output

08-30 19:42:19.130 22008 22042 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:19.130 22008 22008 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 456.877ms
08-30 19:42:19.130 22008 22008 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:19.137 22008 22042 D conversations: update unread count to 2651
08-30 19:42:19.139 22008 22042 D conversations: eta@theta.eu.org: suppressing notification because turned off
08-30 19:42:19.142 22008 22008 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:19.142 22008 22042 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:19.608 22008 22018 I s.conversation: Clamp target GC heap from 119MB to 96MB
08-30 19:42:19.608 22008 22018 I s.conversation: Background concurrent copying GC freed 3922(101KB) AllocSpace objects, 0(0B) LOS objects, 0% free, 95MB/96MB, paused 154us total 477.449ms
08-30 19:42:19.608 22008 22008 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 466.686ms
08-30 19:42:19.608 22008 22008 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:19.608 22008 22042 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 466.510ms
08-30 19:42:19.608 22008 22042 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:19.610 22008 22008 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:19.610 22008 22123 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:19.610 22008 22042 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:20.067 22008 22018 I s.conversation: Clamp target GC heap from 119MB to 96MB
08-30 19:42:20.067 22008 22018 I s.conversation: Background concurrent copying GC freed 3886(155KB) AllocSpace objects, 0(0B) LOS objects, 0% free, 95MB/96MB, paused 115us total 458.027ms
08-30 19:42:20.067 22008 22008 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 457.452ms
08-30 19:42:20.067 22008 22008 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:20.067 22008 22123 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 457.314ms
08-30 19:42:20.067 22008 22123 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:20.067 22008 22042 I s.conversation: WaitForGcToComplete blocked Alloc on HeapTrim for 457.279ms
08-30 19:42:20.067 22008 22042 I s.conversation: Starting a blocking GC Alloc
08-30 19:42:20.071 22008 22008 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:20.117 22008 22042 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:20.250 22008 22019 I s.conversation: Waiting for a blocking GC Alloc
08-30 19:42:20.532 22008 22018 I s.conversation: Clamp target GC heap from 119MB to 96MB

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Closing in favour of #3896 (large heap support), and the memory leak PRs #3898 & #3897 (which I suspect were the real causes)