Crash when filling Android related info on API < 16

[Syhids]

I got this crash on my application:

Fatal Exception: java.lang.NoSuchFieldError: android.app.ActivityManager$MemoryInfo.totalMem
       at io.sentry.android.event.helper.AndroidEventBuilderHelper.getContexts(AndroidEventBuilderHelper.java:124)
       at io.sentry.android.event.helper.AndroidEventBuilderHelper.helpBuildingEvent(AndroidEventBuilderHelper.java:82)
       at io.sentry.SentryClient.runBuilderHelpers(SentryClient.java:112)
       at io.sentry.SentryClient.sendEvent(SentryClient.java:169)
       at io.sentry.Sentry.capture(Sentry.java:177)
       ...

Seems like MemoryInfo.totalMem was included on API 16

I'll work on a PR :)