Use FontManager Java API in tests.

Thanks to commit 84383b5e, now Shell has
UPDATE_FONTS permission and we can use it in instrumentation tests via
UiAutomation#adoptShellPermissionIdentity().

Bug: 186966067
Test: atest UpdatableSystemFontTest
Change-Id: Idd2043f53d95b7c904d563dd3ef1d1846841c0d6