bugfix: fix the 'RootContext.DEFAULT_BRANCH_TYPE' is wrong in some cases

[wangliang181230]

Ⅰ. Describe what this PR did

bugfix: fix the RootContext.DEFAULT_BRANCH_TYPE is wrong in some cases.
修复BUG：在部分情况下，RootContext.DEFAULT_BRANCH_TYPE的值有误。

Cases:

1. springboot项目中，使用配置启用XA自动代理时，而不是使用@EnableAutoDataSourceProxy启用XA自动代理。（这种情况下，XA代理不生效）
2. 手动创建XA代理，并禁用自动代理功能时。（这种情况下，XA代理生效，但是RootContext.DEFAULT_BRANCH_TYPE却是AT）

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #3214 into develop will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@              Coverage Diff              @@
##             develop    #3214      +/-   ##
=============================================
- Coverage      51.05%   51.04%   -0.02%     
  Complexity      3258     3258              
=============================================
  Files            609      609              
  Lines          20023    20027       +4     
  Branches        2499     2501       +2     
=============================================
  Hits           10222    10222              
- Misses          8784     8787       +3     
- Partials        1017     1018       +1     

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/io/seata/core/context/RootContext.java	50.94% <0.00%> (-6.75%)	19.00 <0.00> (ø)
...ation/datasource/AutoDataSourceProxyRegistrar.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ion/datasource/SeataAutoDataSourceProxyAdvice.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...n/java/io/seata/rm/datasource/DataSourceProxy.java	42.85% <100.00%> (+0.92%)	12.00 <0.00> (ø)
...a/io/seata/rm/datasource/xa/DataSourceProxyXA.java	84.61% <100.00%> (+0.61%)	8.00 <0.00> (ø)
...ta/spring/boot/autoconfigure/StarterConstants.java	100.00% <100.00%> (ø)	1.00 <0.00> (ø)

[funky-eyes]

LGTM

[xingfudeshi]

LGTM.