Switch ActivityRecord to use static imports.

- Per comments from ogunwale,jjaggi on ag/2016938, static imports
  are preferred when possible, and are part of Android's tacit style guide.

Bug: 36662390
Test: Covered by existing unit tests.

Change-Id: Ic3897dd7a4a9cb665fc3d8c7a171f74494f02a2e