Update minimum sdk version to 8 (FROYO)

[gryphon2411]

Update library required system minimum API level from API 15
(ICE_CREAM_SANDWICH_MR1) to API 8 (FROYO).

This update will let this library to be available for usage in many more android projects which require lower API level than 15.

When attempting to decrease the minSdkVersion attribute value to < 8, (for example, 7) the following gradle build error occurs:

Error:Execution failed for task ':library:processDebugAndroidTestManifest'.
> java.lang.RuntimeException: Manifest merger failed : uses-sdk:minSdkVersion 7 cannot be smaller than version 8 declared in library [com.android.support.test:runner:0.4]
	Suggestion: use tools:overrideLibrary="android.support.test" to force usage

This is the lowest API level available without any changes to either source code or dependencies.
This update passes all instrumented tests.

[codecov-io]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master    #94   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      271    271           
=======================================
  Files            22     22           
  Lines           659    659           
  Branches         57     57           
=======================================
  Hits            659    659

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[passsy]

We're not going to official support such old versions.

Use this as workaround in your AndroidManifest

    <!-- override the min sdk version -->
    <uses-sdk tools:overrideLibrary="net.grandcentrix.tray" />

We're considering 14 as min sdk for the future. Everything else is out of scope. Even the support library will drop support for older versions