Fixup inconsistency due to change in Plugin loading requirements.

Some time ago plugin loading was changed to always occur with the
thread in kNative (see commit 3a256b1d3689). This adds asserts that
to plugin loading that the state is correct, fixes an incorrect
lock-annotation and fixes a test plugin that was missed.

Test: ./test.py --host
Test: atest CtsJdwpTunnelHostTestCases
Test: atest CtsJvmtiAttachingHostTestCases
Change-Id: Ib27ab58e9a357fa308a33da13a86e8d65575e9f4
3 files changed