Integration test for updatable system font.

This test:
(1) signs a font file with the test key.
(2) side-loads the test cert to the device under test.
(3) verifies that the signed font file can be installed.

The device must be rootable for doing step (2).

Bug: 176939176
Test: atest UpdatableSystemFontTest
Change-Id: I7a9b614aa3c77589c3495b663cb76056ba657006